Union leaders at Rio Tinto Alcan's Quebec smelter are accusing managers of hiring illegal replacement workers after locking out hundreds of unionized employees on Sunday.

Some 780 employees at the Alma, Que. plant were locked out after their contract expired at midnight Sunday, following weeks of negotiations.

Rio Tinto Alcan said it would cut production by one third at its 438,000 tonne smelter in the Saguenay region, as long as the lockout was in effect.

Union leaders at the Syndicat des Métallos d'Alma say they suspect the company is using illegal replacement workers to continue production.

Union representatives said they watched helicopters coming and going from the smelter on Sunday with numerous passengers on board.

Rio Tinto Alcan said about 200 managers will work at the plant during the lockout.

When running at full capacity, the smelter requires nearly 1,000 workers and managers.

The Syndicat des Métallos d'Alma said it will ask Quebec's Labour Ministry to send investigators to check whether provincial labour laws are being respected.

Contract talks unproductive

The smelter's union rejected Rio Tinto Alcan's final contract offer on Friday, capping weeks of negotiations that started in early October.

Union officials had a strike mandate before the talks collapsed and had warned that a work stoppage could be imminent.

The sides in the dispute haven't been able to agree on conditions related to subcontracted labour.

Local leaders urge reconciliation

The lockout could have a major financial impact in Alma, where natural resources industries employ the majority of residents. Alma Mayor Marc Asselin urged both sides to consider mediation sooner than later.

"Talk to each other and find a solution, because this is a conflict that could do a great deal of harm," Asselin said in an interview with Radio-Canada.

"Rio Tinto is a major employer. If we stop production at a plant like the one in Alma, it has enormous economic repercussions."

Rio Tinto Alcan said it will shut down 144 out of 432 reduction cells at the smelter, and will continue to operate the remaining capacity until further notice.

The Alma facility hosts one of Rio Tinto Alcan's most important North American aluminum smelters.